feat: AWS Amplify build spec + deployment docs for docs.grith.ai

[djf73]

Summary

Closes P1 #5 in work/findings/launch-readiness-followup-2026-05-18.md. The docs site has existed for months but docs.grith.ai is not deployed anywhere (HTTP 000, DNS doesn't resolve).

Per your instruction, this wires up AWS Amplify (same provider as grith.ai). The PR is the code half — amplify.yml + setup docs. The console-side half (connecting the repo, adding the custom domain) is your ~5 minutes of clicking; the README walks through it step-by-step.

What's in this PR

amplify.yml (new)

Single-app build spec — grith-docs is a static Next.js export, not a monorepo wrapper. Mirrors the grith-website pattern minus the env-var injection (the docs site has no runtime secrets).

preBuild:    nvm install 22 / npm ci
build:       npm run build      # gen:llms → next build → gen:pagefind
baseDirectory: out
cache:       node_modules + .next/cache

README.md — new "Deployment to docs.grith.ai" section

• Table of canonical Amplify settings
• One-time console steps: connect GitHub repo, pick main, confirm auto-detected build spec, add custom domain, point CNAME, ACM provisions TLS
• Ongoing-operations notes: push-to-deploy on main, optional PR previews, log location

Verified

• python3 -c 'import yaml; yaml.safe_load(open("amplify.yml"))' parses.
• npm run build end-to-end clean locally: 146 pages indexed by Pagefind, search index written to out/pagefind/.
• After merge — you connect the GitHub repo to Amplify per the README, deployment runs, custom domain docs.grith.ai resolves with valid TLS. Then we close P1 #5 in the audit followup.

Your next steps (post-merge)

1. AWS Amplify console → Create new app → Host web app → GitHub.
2. Authorise the AWS Amplify app on the grith-ai org (one-time).
3. Pick grith-ai/grith-docs, branch main.
4. Confirm Amplify auto-detected amplify.yml.
5. Save + deploy. First build ~3-5 min.
6. Domain management → Add domain → docs.grith.ai. Amplify gives you a CNAME target.
7. Add the CNAME in your DNS provider for docs.grith.ai.
8. ACM auto-provisions TLS once DNS resolves.

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